Springtime Atmospheric Responses to North Atlantic SST Anomalies in Idealized GCM Experiments: Northern Hemisphere Circulation and North American Precipitation

In this study, a series of experiments using idealized sea surface temperatures (SST), land and orography are performed to examine the interactions between the Atlantic Multidecadal Oscillation (AMO), continents and major orography. Three sets of experiments are done using an increasingly realistic surface boundary (aqua-planet, land without orography and land with orography) and run using perpetual equinox conditions. For each land surface boundary, the model is forced with a zonally symmetric SST, with additional experiments with an imposed positive or negative SST anomalies in the North Atlantic. The experiments are then compared to determine how these forcings interact and what factors may contribute to the observed atmospheric responses to the AMO. It was found that there are strong nonlinearities in how the atmosphere responds to the warm and cold North Atlantic SST anomalies. During the warm phase of the AMO (warm SST anomaly), there are strong responses in circulation in the Northern Hemisphere and precipitation over North America. When continents are included in the model, these responses are generally enhanced. Orography, on the other hand, acts to weaken the previous forced responses. In contrast, the mid-latitude response during the cold phase of the AMO (cold SST anomaly) is far weaker. We found that land still forces strong mid-latitude responses in both circulation and North American precipitation. However, orography offsets the effects of the land, further altering the response pattern to the cold SST anomaly. These complex interactions between the SST anomalies, land and orography act to produce the generally weak springtime patterns shown in previous studies about the AMO. Furthermore, the modeled details of land and orography interactions and their effects on the responses to the SST anomalies allow for a better understanding of how observed patterns in the AMO responses develop and what may cause variations of those responses. iv Acknowledgements I would like to thank my dissertation adviser, Dr. Qi Hu, for helping to guide me through the research presented in this dissertation. I would also like to thank my committee members, Drs. putting up with me and going along with my insanity as I worked on this dissertation, particularly near the finish. My family provided much support through my time in graduate school (and before) and I would like to thank them for all that they have done for me. Finally, I would like to thank Megumi Watanabe for being there when I needed someone to talk …